At part load on the engine, the exhaust gas quantity decreases, which, with an unaltered turbine inlet cross-section, leads to a reduction in the boost pressure. In consequence, the engine receives too little air so that the exhaust gas temperature and the danger of incomplete combustion increase. In order to be able to travel in this condition without malfunctioning, it is necessary to be able to adapt the turbine inlet cross-section while running so that a reasonably constant boost pressure and smokeless operation over the whole control range are maintained.
The slide ring arrangement described above is limited to radial turbines. The recognisably thin slide ring is provided with an uncontoured end-surface. If the ring protrudes into the flow duct during operation, a strong flow separation with associated losses is unavoidable.